Fiber optical illumination system

ABSTRACT

A fiber optical illumination delivery system, which is effective in reducing the effects of source coherence. The system preferably utilizes either a single bundle of optical fibers, or serial bundles of optical fibers. In the single bundle embodiment, the differences in optical lengths between different fibers of the bundle is preferably made to be equal to even less than the coherence length of the source illumination. In the serial bundle embodiment, the fibers in the other bundle are arranged as groups of fibers of the same length, and it is the difference in lengths of these groups which is made equal to, or even more preferably, less than the overall difference in length between the shortest and the longest fibers in the other bundle. Both of these fiber systems enable construction of illumination systems delivering a higher level of illumination, but without greatly affecting the coherence breaking abilities of the system, thus enabling a generally more applicable system to be constructed.

FIELD OF THE INVENTION

The present invention relates to the field of fiber optical systems forillumination of objects to be imaged, and especially to ways of reducingspeckle effects arising from the coherence of the illumination sourceused.

BACKGROUND OF THE INVENTION

The nature of a laser beam, and especially its coherent nature, presentsa number of problems when used as an illuminating source in applicationsrequiring a uniform illuminating flux over the inspected area, such asis required, for instance, in a wafer inspection system:

-   (i) Interference of light in the illumination optics creates    non-uniformity in the illumination field.-   (ii) Interference of the illuminated light by the structured pattern    on the wafer creates artifacts in the image.-   (iii) Surface roughness creates speckle, that generates    non-uniformity in the image.-   (iv) The laser beam itself is generally not uniform. Using the laser    beam directly as a light source creates non-uniform illumination.

In order to overcome items (i) to (iii) above, the effects of thecoherent nature of the laser beam must be reduced and preferablyeliminated completely. This process is known as coherence breaking.

There are two definitions related to the coherence of a laser beam:

-   (a) Spatial coherence, which is the phase relation between each    spatial point in the laser beam spot. This allows different points    in the spot to interact with each other in a destructive or    constructive manner when the spot is illuminating a cyclic pattern    or a rough surface. This quality depends mainly on the mode of the    beam. For instance in the basic mode (TEM₀₀) the spatial coherence    is defined by the Gaussian profile of the beam.-   (b) Temporal coherence, which is a measure of the time or the    transit distance (the time multiplied by the speed of light in the    medium concerned) over which the phase of the beam can be defined.    This parameter depends on the type of laser and its spectral    bandwidth. Thus, for instance, for the second harmonic of a Nd:YAG    laser at 532 nm, the coherence length is about 8 mm in free space.

There are a number of methods described in the prior art for overcomingcoherence effects in using laser illumination. Reference is made to thearticles “Speckle Reduction” by T. S. McKecknie, pp. 123-170 in Topicsin Applied Physics, Vol. 9, Laser Speckle and Related Phenomena, editedby J. C. Dainty, Springer Verlag (1984), “Speckle reduction inpulsed-laser photography” by D. Kohler et al., published in OpticsCommunications, Vol. 12, No. 1, pp. 24-28, (September 1974) and “Specklereduction with virtual incoherent laser illumination using modifiedfiber array” by B. Dingel et al., published in Optik, Vol. 94, No. 3,pp. 132-136, (1993), and to U.S. Pat. No. 6,369,888 to A. Karpol et al.,for “Method and Apparatus for Article Inspection including SpeckleReduction”, the disclosures of all of which are herein incorporated byreference, each in its entirety.

The above-mentioned prior art solutions to the problem of coherencebreaking variously have specific disadvantages, and it is an object ofthe present invention to attempt to overcome some of these advantages.

SUMMARY OF THE INVENTION

The present invention seeks to provide a new fiber optical illuminationdelivery system, which is effective in reducing the speckle effectsarising from source coherence. The system preferably utilizes either asingle bundle of optical fibers, or serial bundles of optical fibers,according to the various preferred embodiments of the present invention.The single bundle embodiment differs from prior art systems in that thedifferences in optical lengths between different fibers of the bundle ispreferably made to be equal to or more preferably less than thecoherence length of the source illumination. This preferred embodimentenables construction of an illumination system delivering a higher levelof illumination, but without greatly affecting the coherence breakingabilities of the system, thus enabling a generally more applicable andcost-effective system to be constructed.

The serial bundle embodiment differs from prior art systems in that inthe bundle comprising the fibers, where in the prior art systems, thedifferences in lengths of the fibers therein is made equal to theoverall difference in length between the shortest and the longest fibersin the other bundle, according to a preferred embodiment of thisinvention, there are arranged groups of fibers of the same length, andit is the difference in lengths of these groups which is made equal to,or even more preferably, less than the overall difference in lengthbetween the shortest and the longest fibers in the other bundle. Thispreferred embodiment also enables construction of an illumination systemdelivering a higher level of illumination, but without greatly affectingthe coherence breaking abilities of the system, thus enabling agenerally more applicable system to be constructed.

There is thus provided in accordance with a preferred embodiment of thepresent invention, an optical system for reducing the coherence of abeam for illumination of an object, comprising a source of at leastpartially coherent illumination, at least part of which has acharacteristic coherence length, and at least one fiber optics bundlecomprising a plurality of optical fibers, at least some of which havediffering optical lengths, at least some of the fibers of differingoptical length having differences in optical lengths therebetween whichare less than the characteristic coherence length.

In the above system, the source of at least partially coherentillumination may preferably be a laser source, and the coherentillumination may have spatial coherence or temporal coherence or both.To reduce spatial coherence, the plurality of optical fibers in the atleast one fiber optics bundle are preferably randomly ordered.Furthermore, a diffusing element may be used for spatial mixing of thebeam. The optical system may also comprise an optical element positionedsuch that it is operative to direct the illumination from any point ofthe beam into essentially each of the plurality of fibers.

According to yet another preferred embodiment of the present invention,in the above described optical system, the differences in opticallengths being less than the characteristic coherence length, results ina bundle having reduced transmission losses.

In accordance with still another preferred embodiment of the presentinvention, the illumination beam comprises pulses having acharacteristic length, and the bundle is operative to stretch the lengthof the pulses.

There is further provided in accordance with still another preferredembodiment of the present invention, an optical system for reducing thecoherence of a beam for illumination of an object, comprising a sourceof at least partially coherent illumination, at least part of theillumination having a characteristic coherence length, a first fiberoptics bundle comprising a plurality of optical fibers, at least some ofwhich have differing optical lengths, at least some of the fibers ofdiffering optical length having differences in optical lengthstherebetween which are less than the characteristic coherence length,and a second fiber optics bundle disposed serially with the firstbundle, comprising a plurality of groups of optical fibers, each groupof fibers comprising fibers of essentially the same length, and whereinat least some of the group of fibers have differing optical lengths, atleast some of the groups of fibers having differences in optical lengthstherebetween which are at least equal to the sum of all of the opticallength differences of the fibers in the first bundle.

In the above-described embodiment, each of the groups may haveessentially the same number of fibers, or alternatively and preferably,the number of fibers in each of the groups may increase according to theoptical length of the group, and even more preferably, the number offibers in each group may generally be proportional to the length of thegroup.

The bundles may be arranged serially such that the beam for illuminationof the object is initially incident on the first bundle or alternativelyand preferably, the beam for illumination of the object is initiallyincident on the second bundle. In either case, according to furtherpreferred embodiments of this invention, an optical element ispositioned between the bundles such that it is operative to directillumination from any point of the output of the first bundle ontoessentially each point of the input of the second bundle.

In the above system, the source of at least partially coherentillumination may preferably be a laser source, and the coherentillumination may have spatial coherence or temporal coherence or both.To reduce spatial coherence, the plurality of optical fibers in the atleast one fiber optics bundle are preferably randomly ordered.Furthermore, a diffusing element may be used for spatial mixing of thebeam.

In accordance with still a further preferred embodiment of the presentinvention, there is also provided a method of reducing the transmissionloss in a fiber optical bundle for reducing the coherence of lighttransmitted therethrough, at least part of which light has acharacteristic coherence length, the method comprising the steps ofproviding at least one fiber optical bundle comprising a plurality ofoptical fibers, at least some of which have differing optical lengths,and arranging the lengths of the plurality of optical fibers such thatat least some of the fibers of differing optical lengths havedifferences in optical length therebetween generally less than thecharacteristic coherence length.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings in which;

FIG. 1 is a schematic illustration of a bright field object inspectionsystem, utilizing a laser source and a fiber optical delivery bundle,constructed and operative according to a preferred embodiment of thepresent invention;

FIG. 2 is a schematic drawing of a fiber optical delivery bundle,according to a preferred embodiment of the present invention, such asthat used in FIG. 1;

FIGS. 3A to 3E schematically show various preferred embodiments of fiberbundle applications, according to further preferred embodiments of thepresent invention;

FIG. 3A is a graphical illustration of the transmission and thecoherence reduction factor of a single fiber optical bundle, such asthat shown in the embodiment of FIG. 1, as a function of fiber opticallength difference divided by the coherence length of the source;

FIG. 3B is a schematic illustration of a double bundle fiber opticalillumination system, according to a preferred embodiment of the presentinvention;

FIGS. 3C and 3D respectively illustrate schematically two embodiments ofa first bundle of a double bundle illumination system, such as that ofFIG. 3B, according to another preferred embodiment of the presentinvention, in which the bundle is made up of groups of fibers of thesame length; and

FIG. 3E is a schematic drawing of the second bundle of fibers of thepreferred embodiment of FIG. 3B, in which each of the fibers is of adifferent optical length, the optical lengths preferably differing bythe coherence length of the light source or less.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to FIG. 1, which is an overall schematic side viewof the complete illumination system of the defect detection apparatus,according to one preferred embodiment of the present invention.According to different preferred methods of operation, three alternativemodes of illumination are provided: Bright Field (BF), Side-illuminatedDark Field (DF) and Orthogonal or Obscured Reflectance Dark Field (ODF).Each mode of illumination is used to detect different types of defectsin different production process steps. For example in order to detect anembedded defect in a transparent layer, such as silicon oxide, BFillumination is preferred. In order to detect a small particle on asurface, DF illumination generally yields better results.

In bright field illumination in general, the illumination is incident onthe sample through the same objective lens as is used for viewing thesample. Reference is now made to FIG. 1, which shows a bright fieldilluminating laser source 300 delivering its output beam 15 into anoptical delivery fiber bundle 21, preferably by means of a laser tofiber coupler 150. This optical fiber bundle 21 is required for the dualpurposes of providing uniform illumination on the sample and forcoherence breaking of the laser illumination, as will be expoundedfurther hereinbelow. In the preferred embodiment of FIG. 1, only asingle fiber bundle is used, but it is to be understood that a serialfiber bundle solution, as will be shown hereinbelow, could just asreadily have been used. From the output termination of the fiber bundle21, the laser beam is imaged by means of illumination transfer lenses301, 302, onto the objective lens in use 201, which is operative tofocus the illumination onto the wafer plane 100 being inspected.Appropriate alternative objective lenses 201′ can be swung into place onan objective revolver 200, as is known in the microscope arts. Theillumination returned from the wafer is collected by the same objectivelens 201, and is deflected from the illumination path by means of a beamsplitter 202, towards a second beam splitter 500, from where it isreflected through the imaging lens 203, which images the light from thewafer onto the detector 206. The second beam splitter 500 is used toseparate the light going to the imaging functionality from the lightused in the auto-focus functionality, which is directed by means of theauto-focus imaging lens 501 to the auto-focus detector 502.

When conventional dark field illumination is required for the imaging inhand, a dark field side illumination source 231 is used to project therequired illumination beam 221 onto the wafer 100. When orthogonal darkfield, or obscured reflectance dark field illumination is required forthe imaging in hand, an alternative dark field illumination source 230is used to project the required illumination beam 232 via the obscuredreflectance mirror 240 onto the wafer 100 orthogonally from above.

A repetitively pulsed laser source is preferably used in theillumination system of the present invention, though according to otherpreferred embodiments, CW laser illumination may also be used. Inaccordance with the requirements of providing a high brightness lightsource that produces a directionally intense beam of short time durationand at high repetition rates, the third harmonic of a Nd:YAG Laseroutput is preferably used.

Speckle effects with CW lasers is comparatively easy to overcome, sinceit is possible to average the signal while varying the wave front.Several methods are described in the prior art for achieving this. When,however, the imaging process utilizes a single pulse for each acquiredimage, such a method becomes impossible to implement. According tofurther preferred embodiments of the present invention, there areprovided methods whereby the coherence effect of the laser beam isreduced by splitting the laser beam into many beamlets and retardingeach beamlet relative to the previous one in such a way that there is nodefinitive phase difference between them. The laser beam is thus dividedinto many parts, each part having no defined phase coherence with theother parts.

This requirement is insufficient, however, since it is also requiredthat each point in the field of view (FOV) on the sample is illuminatedby all parts of the laser beam. Each part of the beam is coherent orpartially coherent with itself and thus may contribute to the generationof speckle, or to other interference effects that create high contrastartifacts in the image. Since each part of the beam is not coherent withthe other parts of the beam, by ensuring that the FOV is illuminated byall parts of the laser beam, the total effect is averaged. The residualcoherence effect depends on the number of beamlets used. Since eachbeamlet is independent of the others, the interference effect is reducedby the square root of the number of beamlets, assuming that all beamletshave the same intensity contribution. Consequently, the greater thenumber of beamlets, the lower the level of appearance of coherenceartifacts in the image.

According to preferred methods of implementation of this technique, thelaser beam is introduced into a fiber optics bundle, such as the fiberbundle 21 shown schematically in FIG. 1. The fibers in the bundle differin length from each other by distances of the order of the lasercoherence length in the fiber medium, or less. The number of fibers inthe bundle dictates the contrast of the residual coherence effect in theimage. The fiber bundle should preferably be illuminated uniformly. Eachfiber in the bundle must carry more or less the same energy; otherwiseaveraging of the coherence effect will not be efficiently performed.Since the laser beam itself is not uniform and contains high and lowspatial frequency components, the laser beam must be spatially mixedbefore introduction into the fiber. Additionally, the full numericalaperture of the fiber should preferably be filled, since at the far endof the bundle, uniform angular distribution of intensity is required.These latter two requirements do not appear to be fulfilled in the priorart. In the above-referenced article by Dingel et al., although it isstated that Koehler illumination is generated, no arrangement is shownfor spatially mixing the laser beam, nor is there described a specificmethod for ensuring that the incident light is directed such that theNumerical Aperture of each fiber is fully illuminated. Under theconditions shown, each fiber would illuminate randomly, resulting innon-uniform field stop plane intensity, which then would also result innon-uniform illumination at the object plane. Furthermore, in the Dingleet al prior art, it is stated that the proposed array is made of Nfiber-guides in which the length difference of any two fibers is greaterthan the coherence length of the light source. Such an arrangement wouldgenerally result in excessive differences, since it is the opticallength difference and not the absolute length difference of any twofibers which needs to be greater than the coherence length of the light,according to the criteria chosen in the Dingel et al. article. Finally,the illumination system described in this prior art is for atransmissive imaging system.

An implementation of this method, according to a preferred embodiment ofthe present invention, is schematically illustrated in FIG. 2. The laserbeam 15, which can be either a parallel beam or slightly convergent, orslightly divergent impinges onto a diffusing element 16, which,according to alternative and preferred embodiments, can be a regulardiffuser, a holographic diffuser (such as an MEMS) or a micro-lens arrayhaving a numerical aperture that spreads the incident light at therequired angles. The diffused beam, shown schematically in FIG. 2 bymeans of three exemplary rays 17 diffused at the same angle fromdifferent locations in the diffuser, is preferably imaged onto one pointof the end face 20 of the terminal connector 19 of a fiber optics bundle21 by means of a focusing element 18, which can be either a single lens,or, in order to reduce aberations, a multi-element lens. Rays diffusedat different angles from those rays 17 shown in FIG. 2, are imaged ontodifferent points of the end face 20 of the fiber optics bundle 21. Lightfrom all of the included angles at which light is output from thediffuser is thus imaged by means of the focusing element 18 to cover theentire input aperture of the fiber bundle end face 20. The beamtraverses the fiber bundle 21 and is output at the opposite end face 29of the fibers at the output connector 22.

For optimum optical transfer efficiency, the diffusing element 16 ispreferably positioned at the left focal plane of the focusing element18, and the end face 20 of the fiber 21, at the right focal plane of thefocusing element.

The half angle α of the diffusing element, and the focal length f, offocusing element are computed as follows:

“If r is the input beam radius and NA is the numerical aperture of thefiber, then NA=r/f by definition. Thus f=r/NA. Now, if R is the fiberbundle radius than α*f=R. Thus, for a specific input beam diameter andfiber diameter, the focal length and the diffusing angle can be simplycalculated.”

The embodiments generally described in the prior art of the use of afiber bundle to provide coherence breaking have disadvantages, relatingto the effect of transmission losses in the fibers. In order to providegood coherence breaking, the difference in length between any pair offibers of the bundle is described in the prior art as needing to begreater or equal to the coherence length of the light source. As aconsequence, the difference in length between the fibers in the bundleis thus greater or equal to the coherence length times the number offibers in the other bundle. Consequently, according to the criteria ofthe prior art, for a bundle containing hundreds or even thousands offibers, there is an appreciable difference in length between theshortest and longest fibers of the bundle. This results in twodisadvantageous effects in such prior art fiber bundles:

-   (i) Firstly, because of the transmission loses in typically used    fiber materials, the light intensity output from each fiber of the    bundle may be significantly different, falling with increasing fiber    length. However, for the coherence breaking effect to be effective,    there should ideally be only phase or time of flight differences    between the various fiber outputs, and any differences in intensity    contribution degrades the desired coherence breaking effect.-   (ii) Secondly, the longer these differences in length, the longer    the overall length of the bundle, and the longer the overall length    of the bundle, the higher the transmission losses themselves, quite    apart from their effect on the coherence breaking effects. These    transmission losses make the illumination system inefficient and    less cost-effective.

This effect can be illustrated by reference to FIG. 3A, which is agraphical illustration of the outcome of the above-described trade-offin fiber length difference between transmission and coherence breakingefficiency. The results shown in FIG. 3A are for a bundle containing40,000 fibers, and for a fiber having a transmission loss in the UV ofthe order of 0.1 db/m. The two ordinates separately show the bundletransmission and the coherence reduction factor as a function ofDelta/L_(C), where each fiber differs in length by Delta mm., and thecoherence length of the source is L_(C). The transmission is measuredrelative to a bundle having uniform fiber lengths equal to the length ofthe shortest fiber in the variable fiber length bundle. The value ofL_(C) for the example shown is 5 mm.

“For such a 40,000 fiber bundle, the maximum theoretical coherencereduction factor is given by (40,000)^(1/2)=200. As is observed in thegraph, for Delta/L_(c)=1, meaning that the fiber optical lengthdifferences are equal to the coherence length, the coherence reductionfactor is approximately 90, compared to the maximum theoretical 200. Itis to be noted that the coherence reduction factor falls short of itstheoretical value because the increasing insertion loss of eachsuccessive fiber means that the intensity contribution of each separatefiber to the total output is not equal, and the coherence breakingeffect is thus reduced. The transmittance of the bundle, on the otherhand, has fallen to only 0.22 of that of a bundle with Delta/L_(c)=0,i.e. with no length differences, and such a transmission loss isserious.”

If, on the other hand, the fiber optical length difference is reduced toonly 0.4L_(C), the coherence reduction factor is reduced toapproximately 85, which is only a 6% reduction, while the transmissionis increased to approximately 0.45, which is over a 110% increase.

According to these results, there is thus provided, according to apreferred embodiment of the present invention, an illumination deliveryfiber bundle, operative for breaking the coherence of light transmittedtherethrough, in which the differences in lengths of the fibers in thebundle are less than the coherence length of the source. Such a bundle,which compromises slightly on its coherence breaking properties by usingfiber differences less than the coherence length, and thereby gains asubstantial increase in illumination level, thus has significanteconomical advantages over the prior art bundles described above.

The above mentioned embodiments have been generally described in termsof typical pulsed laser sources, such as Nd:YAG lasers, where thecoherence length is generally of the order of a few millimeters. It isevident that in systems using longer coherence length lasers, theproblem is multiplied manyfold. Thus, for instance, a Helium-Neon CWlaser typically has a coherence length of the order of 20 cm, underwhich conditions, the advantages of any of the various embodiments ofthe present invention become even more pronounced.

In order to improve the coherence breaking efficiency, it is known, forinstance from the above-referenced U.S. Pat. No. 6,369,888, that it maybe more economical to use two bundles with a smaller number of fibers ineach, than one bundle with more fibers. If the fiber length differencesin the first bundle exceeds the overall fiber length difference betweenthe shortest and the longest fibers in the second bundle, then theeffective number of fibers taking part in the coherence breaking processis the number of fibers in the first bundle times the number of fibersin the second bundle. This applies if the contribution of light to eachfiber in the second bundle comes from all of the fibers in the firstbundle.

Reference is now made to FIG. 3B, which is a schematic illustration ofan optical arrangement for achieving this result, wherein a secondbundle is provided serially with the first bundle of FIG. 2. From theexit end face 29 of the first bundle 21, three exemplary rays 23propagating at the same angle from different locations in the end face20, are shown being imaged onto the end face 26 of the fibers at theterminal connector 25 of the second fiber optics bundle 27 by means of afocusing element 24, which can be either a single lens, or amulti-element lens. The beam is output from the second fiber bundle 27at the far end face 26 of the fibers at the output connector 28. It isnot necessary that the diameter of the first bundle 21 be the same asthe diameter of the second bundle 27, as shown in the preferredembodiment of FIG. 3B. If the first bundle has a smaller diameter, adiffuser is required at its end to increase the angular distribution oflight from the end, in order to fill the input of the second bundle.

In the embodiment of the double fiber bundle arrangement described inU.S. Pat. No. 6,369,888, the fibers in both bundles are described ashaving different lengths, and the difference in length ΔL between anytwo fibers in one bundle is preferably selected to be greater than thecoherence length of the light source. The difference in length betweenany pair in the other bundle is described as being, preferably largerthan the difference in length between the shortest and the longest fiberin the first mentioned bundle.

However, in addition to the prior art disadvantage described aboveconcerning the effect of the fiber length differences on the totalintensity transmitted by the bundle, there is another disadvantagerelating to the variation in intensity transmitted by the various fibersof the prior art double bundle embodiments. In order to provide goodcoherence breaking with a double bundle configuration, it is importantthat the phase-separated beamlets input to the second bundle, asgenerated by the different lengths of the fibers in the first bundle,should ideally be of equal intensity. Any departure from equal intensityresults in degradation of the coherence breaking effect in the secondbundle, since some of the differently phased output beams will bepreferentially more intense than others, leading to a net residualcoherence effect. In the above-mentioned Karpol et al patent, thedifference in length between any pair of fibers in the first bundle isdescribed as being preferably larger than the difference in lengthbetween the shortest and the longest fiber in the other bundle. Thedifference in length between any pair of fibers of that other bundle isdescribed as being greater than the coherence length of the lightsource, such that the difference in length between the shortest and thelongest fiber in the other bundle is thus greater than the coherencelength of the light source times the number of fibers in the otherbundle. The typical coherence lengths generated by lasers used for suchapplications are of the order of up to a few millimeters. Consequently,according to the criteria of this prior art, there is an appreciabledifference in length between the fibers of the first bundle.

“There is therefore also a second trade-off between two effects, whichoppositely affect the efficiency of the coherence breaking. On the onehand, the differences between the lengths of the fibers in the secondbundle should preferably be more than the coherence length in order togenerate efficient coherence breaking in such a bundle, and on the otherhand, the larger the difference in lengths between the fibers anywherein the double bundle embodiment, the more the coherence breaking in thesecond bundle is degraded because of lack of unity of intensity.”

Furthermore, in the above-mentioned Karpol et al., prior art, it isstated that the difference in length ΔL between any two fibers in onebundle is preferably selected to be greater than the coherence length ofthe light source. This preferred difference in length is longer than theoptical path length in the fiber by a factor N, where N is therefractive index of the core material, such that this method proposesuse of a longer length difference between fibers than is dictated byoptical considerations, even before any incentive to reduce fiber lengthdifferences, as discussed hereinabove.

“Reference is now made to FIGS. 3C and 3E, which respectively illustrateschematically the two bundles of a double fiber bundle delivery system,constructed and operative according to another preferred embodiment ofthe present invention. This embodiment is operative to diminish theabove-described disadvantages of the prior art double fiber bundledelivery system. For the purposes of explaining the operation of thisembodiment, the fiber bundle shown in FIG. 3C is regarded as the inputbundle, denoted 21 in the embodiment of FIG. 3B, and the fiber bundleshown in FIG. 3E is regarded as the output bundle, denoted 27 in FIG.3B, though it is to be understood that this embodiment is equallyoperable with the fibers in either order, if the correct matchingcomponents are provided.”

Considering now the first bundle, in order to generate good coherencebreaking, every fiber should optimally be of a different optical lengthby the sum of all of the optical length differences of the fibers in thesecond bundle. On the other hand, in order to avoid intensity variationeffects from degrading the coherence breaking effect of the secondbundle, equal optical length fibers should ideally be used, but thiswould generate no coherence breaking in the first bundle. There istherefore provided, in accordance with a preferred embodiment of thepresent invention, and as illustrated in FIG. 3C, a compromise bundleconstruction, in which the fibers are divided into groups, each groupcontaining fibers of the same optical length, and each group preferablybeing different in optical length from another group in the bundle bythe sum of all of the optical length differences of the fibers in thesecond bundle. In the embodiment of FIG. 3C, therefore, the fiberswithin each group provide an element of uniformity to the beamletsoutput from the first bundle, while the difference in optical lengthsbetween the groups provides the coherence breaking properties of thelight from the different groups. The correct trade-off between these twoeffects is able to compensate to a large extent for the reduction inefficiency from the coherence breaking effect that would be obtained ifall the fibers were of different optical lengths, but were also lossfree, such that the intensity change effect was not a factor. The extentof the compensation between these two effects is a function of theattenuation per unit length of the fiber used.

According to yet another preferred embodiment of the present invention,instead of each group having the same number of fibers, as a result ofwhich, the longer groups still have a lower light output than theshorter groups, it is possible to ensure that each group has the sametransmitted intensity by varying the number of fibers in each group.Reference is now made to FIG. 3D, which is a schematic drawing of abundle of fibers, according to yet another preferred embodiment of thepresent invention, similar to that shown in FIG. 3C, but in which thenumber of fibers in each group is increased according to the length ofthe group. Even more preferably, the number of fibers in each group ismade generally proportional to the length of the group. In this way, theincreased insertion loss arising in a group because of the additionalfiber length in the group is offset by the increase in the number offibers in that group.

A further advantage in the use of groups of fibers, according to thisembodiment of the present invention, is that the redundancy effect of alarge number of fibers operating in parallel has the effect of smoothingout any production differences which inevitably arise between supposedlyidentical fibers both in optical properties and in targeted cleavedlength.

Reference is now made to FIG. 3E, which is a schematic drawing of thesecond bundle of fibers, according to this preferred embodiment of thepresent invention. In the second bundle, each of the fibers ispreferably of a different optical length, the optical lengths preferablydiffering by the coherence length of the light source or less. Since thetotal overall difference in optical lengths of the Fibers in the secondbundle is determinative in fixing the difference in the optical lengthsbetween the different groups of fibers in the first bundle, and asmentioned above, there is an advantage in keeping the path differencesbetween fibers as short as possible to minimize intensity changesbetween fibers or fiber groups, there is an additional advantage to thefirst bundle parameters if the optical lengths of the fibers in thesecond bundle differ by as little as possible. For this reason,preferred use of fiber optical lengths differing by less than thecoherence length of the light source can be advantageous in the secondbundle, commensurate with achieving sufficient coherence breakage in thecombination. The determination of how much less than the coherencelength to use in a particular beam delivery system is ascertainedaccording to the attenuation constant of the fibers used in the bundles,and in accordance with the above-described trade-off considerations.

According to the above mentioned preferred embodiments of the presentinvention, there is described a system comprising only one bundle of thetype containing the groups of fibers, whether that bundle is positionedin front of or after the bundle containing the single fibers. Accordingto more preferred embodiments of the present invention, in series withthe bundle containing the single generally ungrouped fibers, a pluralityof bundles with groups of fibers can be used, instead of a single suchbundle, such that the illumination system comprises a series of bundlesof fibers, with the groups of fibers and the fibers respectivelyoptimally arranged for good coherence breaking properties and minimaltransmission losses, as expounded hereinabove.

Some examples are now provided to illustrate one preferred embodiment ofFIGS. 3C to 3E quantitatively. Reference is first made to the secondbundle, as shown in FIG. 3E, which has k fibers, where k is preferablyof the order of 1000. The length of a first fiber is L, where L ispreferably of the order of 1 meter. A second fiber is longer than thefirst by L_(c)/N, where L_(c) is the coherence length of the lasersource, typically 6 mm, and N is the fiber core refractive index,generally of the order of 1.5, such that the fiber length difference isof the order of 4 mm. A third fiber is longer than the second also byL_(c)/N, and so on. The sum of all k length differences is thusk×L_(c)/N , which amounts to the order of 4 meters for this preferredexample.

The first bundle, as in FIG. 3C, has a number n of groups of fibers,where n is preferably 10 to 20. Each group contains m, preferably 20 to50, fibers of equal length and equal optical path length. The lengthdifference between each of the groups is equal to or greater than thesum of all of the length differences of the second bundle, which, inthis preferred example, amounts to approximately 4 meters, as obtainedabove. From these numerical examples, the reason for limitingdifferences in fiber lengths to limit transmission loss changes, becomesevident.

The above-described embodiments of the present invention for achievingbeam coherence breaking also result in a solution for a problem relatedto the use of short pulsed lasers in such illumination systems. Suchshort laser pulses, which can typically be as short as only a fewnanoseconds, may have a peak power density so high that the focussedbeam may cause damage to the wafer under inspection. A common methodused to decrease the peak power of a short laser pulse is to stretch thepulse, such that the pulse energy is expended over a longer time, andhence has a lower peak power. Such pulse stretching can be performed bytransmitting the pulse in parallel down several paths of differentoptical path length, and recombining after transit. This is thesituation which exists with the assembly of variable length fibers inthe bundles shown in the embodiments of FIGS. 2 and 3A-3E of the presentinvention, such that the fiber bundles of the present invention are alsoeffective in pulse stretching applications.

To illustrate this application of the preferred embodiments of thepresent invention, the above mentioned numerical example will be used.For the preferred bundle having 20 groups, each different in length by 4meters, a total length difference of 80 meters is generated. The time offlight of light in the medium of the fiber, having a refractive index of1.5, is approximately 5 nsec/meter. Thus the total time of flightdifference for an 80 meter bundle is approximately 400 nsec. The effectof the bundle is thus to generate pulse stretching from the typicallyfew nanosecond pulse lengths emitted by the laser, to about two ordersof magnitude longer, with the concomitant reduction in potential beamdamage. For at least one bundle some or all of the optical pathdifferences between fibers is less than the beam coherence length.

A particular feature of a preferred embodiment of the present inventionis that the system includes a second fiber optic bundle, within whichthe optical path length difference between each pair of fibers is lessthan or equal to the coherence length of the light beam being employedby the system.

It is appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather the scope of the present inventionincludes both combinations and subcombinations of various featuresdescribed hereinabove as well as variations and modifications theretowhich would occur to a person of skill in the art upon reading the abovedescription and which are not in the prior art.

1. An optical system for reducing the coherence of a beam for illumination of an object, comprising: a source of at least partially coherent illumination, at least part of said illumination having a characteristic coherence length; a first fiber optics bundle comprising a plurality of optical fibers, at least some of which have differing optical lengths, at least some of said fibers of differing optical length having differences in optical lengths therebetween which are less than said characteristic coherence length; and at least a second fiber optics bundle disposed serially with said first bundle, comprising a plurality of groups of optical fibers, each group of fibers comprising fibers of essentially the same length, and wherein at least some of said group of fibers have differing optical lengths, at least some of said groups of fibers having differences in optical lengths therebetween which are at least equal to the sum of all of the optical length differences of said fibers in said first bundle.
 2. An optical system according to claim 1 and wherein said at least a second fiber optics bundle comprises a plurality of serially disposed fiber optics bundles, each comprising a plurality of groups of optical fibers.
 3. An optical system according to claim 1 and wherein each of said groups comprises essentially the same number of fibers.
 4. An optical system according to claim 1 and wherein the number of fibers in each of said groups increases according to the optical length of said group.
 5. An optical system according to claim 4 and wherein the number of fibers in each group is generally proportional to the length of said group.
 6. An optical system according to claim 1 and wherein said bundles are arranged serially such that said beam for illumination of said object is initially incident on said first bundle.
 7. An optical system according to claim 6 and wherein each of said first bundle and said second bundle have an input and an output; and also comprising an optical element positioned between said first bundle and said second bundle such that it is operative to direct illumination from any point of said output of said first bundle onto essentially each point of said input of said second bundle.
 8. An optical system according to claim 1 and wherein said bundles are arranged serially such that said beam for illumination of an object is initially incident on said second bundle.
 9. An optical system according to claim 8 and wherein each of said first bundle and said second bundle have an input and an output; and also comprising an optical element positioned between said second bundle and said first bundle such that it is operative to direct illumination from any point of said output of said second bundle onto essentially each point of said input of said first bundle.
 10. An optical system according to claim 1 and wherein said source of at least partially coherent illumination is a laser source.
 11. An optical system according to claim 1 and wherein said at least partially coherent illumination has at least one of spatial coherence and temporal coherence.
 12. An optical system according to claim 11 and wherein said plurality of optical fibers in said first fiber optics bundle are randomly ordered, such that said spatial coherence is reduced.
 13. An optical system according to claim 1 and also comprising a diffusing element for spatial mixing of said beam.
 14. An optical system according to claim 1 and wherein said differences in optical lengths of some of said plurality of optical fibers in said first bundle, being less than said characteristic coherence length, reduces transmission losses in said system. 